Temperature compensated amplifier

Abstract

A temperature compensated amplifier with variable gain and to a radio device having an amplifier according to it. The gain control and temperature compensation of a differential amplifier ( 210 ) are implemented by a control circuit ( 220 ), which has a balanced and differential output (V 1 , V 2 ). The output voltage of the control circuit, or the control voltage, is arranged to be proportional to difference between two source currents (I GT1 , I GT2 ), which difference can be varied on both sides of zero. The output of the control circuit is connected to the bases of the differential pair (Q 1 , Q 2 ) of the variable gain amplifier, whereupon the ratio of the output current (i out ) to the input current (i in ) of the pair becomes dependent on the control voltage. This is arranged to be proportional to the absolute temperature, too. A temperature change then changes the control voltage the more the higher the control voltage is. The temperature coefficient is adjusted so that the effect of the temperature on the current difference of the differential pair of the variable gain amplifier is compensated. Owing to differential type control circuit the current gain with temperature compensation can be varied in the whole range 0 . . . 1.

1. A temperature compensated amplifier with variable gain comprising at least one differential pair, total current of which being arranged to be dependent on input signal of the amplifier and one collector current of which being output signal of the amplifier, the amplifier further comprising a control circuit to change a control of transistors of the differential pair
 so that the ratio of said one collector current to the total current, or current gain, changes corresponding to changes in a gain control signal and 
 as a function of the temperature so that the effect of the temperature change on the collector currents inside the differential pair is substantially compensated, wherein; 
 the control circuit has a differential output, first terminal of which is connected to base of first transistor of the differential pair and second terminal of which is connected to base of second transistor of the differential pair, 
 the control circuit comprises a first and a second current source, the current difference of which sources can be varied on both sides of zero by means of said gain control signal, 
 output voltage of the control circuit is proportional to said current difference, the output voltage can thus be varied on both sides of zero to set the current gain to be more or less than 0.5, 
 the currents of the first and second current source are arranged to be dependent on the temperature, wherein a change in the output voltage of the control circuit being due to a temperature change, depends on both temperature difference and said current difference and causes changes in the collector currents of the differential pair, which changes have opposite sign compared with the immediate effect of said temperature change on said collector currents inside the differential pair, and 
 the control circuit is further arranged such that above-mentioned changes with opposite signs in each collector current of the differential pair are substantially equal regarding their absolute values. 
 
   
   
     2. The amplifier according to  claim 1 , the currents (I GT1 , I GT2 ) of the first and second current sources depending linearly on the absolute temperature, wherein a change in the output voltage of the control circuit being due to a temperature change, is linearly proportional both to temperature difference and to the value of said current difference itself. 
   
   
     3. The amplifier according to  claim 1 , the control circuit comprising an operational amplifier with a differential output, and the first current source being connected from the non-inverting input of the operational amplifier to the ground and the second current source being connected from the inverting input of the operational amplifier to the ground, from the non-inverting input further being connected a resistor to a reference voltage and another resistor to first terminal of the differential output of the amplifier and from the inverting input further being connected a resistor to the reference voltage and another resistor to second terminal of the differential output of the operational amplifier, which all four resistors have the same resistance, wherein the output voltage of the control circuit is product of said resistance and said current difference. 
   
   
     4. The amplifier according to  claim 3 , wherein the arrangement of the control circuit such that the changes with opposite signs in each collector current of the differential pair are substantially equal regarding their absolute values, is implemented by choosing a suitable value for said resistance. 
   
   
     5. The amplifier according to  claim 1 , having two differential pairs of said type, e.g. first and second pair, connected parallel such that the bases of first transistors of these pairs are connected to the first output terminal of the control circuit and the bases of second transistors of these pairs are connected to the second output terminal of the control circuit, and said input signal is arranged to be fed to a separate third differential pair, the collector of first transistor of which third pair being connected to the emitters of the transistors of the first pair and the collector of second transistor of the third pair being connected to the emitters of the transistors of the second pair. 
   
   
     6. A radio device, a transmitter of which has a temperature compensated amplifier with variable gain to set the transmitting power and to keep the transmitting power independent of the temperature, the amplifier comprising at least one differential pair, total current of which being arranged to be dependent on input signal of the amplifier and one collector current of which being output signal of the amplifier, the amplifier further comprising a control circuit to change a control of transistors of the differential pair
 so that the ratio of said one collector current to the total current, or current gain, changes corresponding to changes in a gain control signal and 
 as a function of the temperature so that the effect of the temperature change on the collector currents inside the differential pair is substantially compensated, wherein; 
 the control circuit has a differential output, first terminal of which is connected to base of first transistor of the differential pair and second terminal of which is connected to base of second transistor of the differential pair, 
 the control circuit comprises a first and a second current source, the current difference of which sources can be varied on both sides of zero by means of said gain control signal, 
 output voltage of the control circuit is proportional to said current difference, the output voltage can thus be varied on both sides of zero to set the current gain to be more or less than 0.5, 
 the currents of the first and second current source are arranged to be dependent on the temperature, wherein a change in the output voltage of the control circuit, being due to a temperature change, depends on both temperature difference and said current difference and causes changes in the collector currents of the differential pair, which changes have opposite sign compared with the immediate effect of said temperature change on said collector currents inside the differential pair, and 
 the control circuit is further arranged such that the above-mentioned changes with opposite signs in each collector current of the differential pair are substantially equal regarding their absolute values.